This invention relates to RF load and source pull testing of medium and high power RF transistors and amplifiers using remotely controlled electro-mechanical impedance tuners.
Modern design of high power RF amplifiers and mixers, used in various communication systems, requires accurate knowledge of the active device's (microwave transistor's) characteristics. In such circuits, it is insufficient for the transistors, which operate in their highly non-linear regime, close to power saturation, to be described using non-linear numeric models.
A popular method for testing and characterizing such microwave components (transistors) in the non-linear region of operation is “load pull”. Load pull is a measurement technique employing microwave tuners and other microwave test equipment (FIG. 1), such as signal source (1), input and output tuner (2, 4), power meter (5) and test fixture (3) which includes the DUT. The tuners and equipment are controlled by a computer (6) via digital cables and communication (7, 8, 9). The microwave tuners are used in order to manipulate the microwave impedance conditions under which the Device Under Test (DUT, or transistor) is tested (see ref. 1); this document refers hence to “impedance tuners”, (see ref. 2), in order to make a clear distinction to “tuned receivers (radios)”, popularly known elsewhere also as “tuners”, because of the included frequency tuning circuits.
Electro-mechanical impedance tuners (FIG. 2) in the frequency range of interest include, a slabline (26) with a center conductor (27) and one or more mobile carriages (28) which carry a motor (24), a vertical axis (23) and control the vertical position (29) of a reflective (tuning) probe (22). The carriages are moved horizontally by additional motors and gear. The signal enters into one, the test port (201), and exits from the other, the idle port (202). The entire mechanism is, typically, integrated in a solid housing (203) since mechanical precision is of highest importance.
The typical configuration of the reflective (tuning) probe inside the slabline is shown in FIGS. 3 and 4; in general, a slotted transmission airline (34) includes a number of parallel tuning elements (31) also called “tuning” probes or slugs, which are coupled capacitively (gap [D]) with the center conductor to an adjustable degree (34), depending from very low (when the probe is pulled up, or ‘withdrawn’) to very strong (when the probe is within corona discharge (spark) distance from the center conductor); tuning (‘reflective’) probes are different than ‘signal sampling’ probes, which are loosely coupled with the center conductor and not grounded, because they must transfer the detected signal power to adjacent measurement instruments; when the reflective probes approach (34) the center conductor (33) of the slabline (34) and moved along the axis (35) of the slabline, they modify the amplitude and phase of the reflection factors, covering parts or the totality of the Smith chart (the normalized reflection factor plan). The relation between reflection factor and impedance is given by GAMMA=(Z−Zo)/(Z+Zo), wherein Z is the complex impedance Z=R+jX and Zo is the characteristic impedance. A typical value used for Zo is Zo=50 Ohm (see ref. 3).
Up to now such metallic probes (slugs) have been made in a block (parallelepiped) form (31) with a concave bottom (402) (FIG. 4), which allows capturing, when approaching the center conductor (401), the electric field, which is concentrated sidewise in the area which is closest between the center conductor and the grounded sidewalls of the slabline, and reflect most of the signal power back. This field capturing allows creating high and, through accurate positioning of the probe, controllable reflection factors. Contact of the probes with the sidewalls (40, 44) (FIG. 4) is critical. It can be either capacitive (44) (FIG. 4b) or galvanic (40) (FIG. 4a). If the contact is capacitive (FIG. 4b), the surface of the probes facing the slabline walls and/or the sidewalls of the slabline must be electrically insulated and perfectly smooth and parallel to each other. Insulation can be done using chemical process such as anodization. Nevertheless capacitive contact means extreme requirement in sidewall planarity and straightness to keep the capacitive contact constant for the whole length and depth of the slabline as the probe travels.
Galvanic contact (FIG. 4a) is safer, but requires a spring loaded mechanism to allow for continuous pressure of the probe on the sidewalls and reliable ground sliding contact. The springing mechanism (41) of probe 4a) is created by machining a horizontal hole (42), parallel to the center conductor of the slabline, into the body of the probe and leave a thin strip of metal at the sides to act as spring. Probe 4b) instead can be massive (45).